Carpets generally comprise a primary backing structure, face yarn, a binder and, in many cases, a secondary carpet backing. Face yarn penetrates the primary backing structure to form tufts projecting from one side, providing a pile surface, and stitches on an opposite side. Binder is present on the stitched side, encapsulating and adhering stitches to the backing structure to anchor the tufts. Secondary carpet backings normally are adhered to the stitched side with the binder. The binder typically comprises more than 80 percent by weight of a ground inorganic solid such as calcium carbonate in a styrene butadiene or ethylene vinyl acetate latex.
Carpets are typically fabricated by tufting face yarn through a primary backing structure with reciprocating needles that carry face yarn back and forth through the structure to form the tufts and stitches, applying a binder formulation, usually as an inert particulate-filled aqueous latex of an organic polymer, to the stitched side, and curing the binder by heating to drive off water or other liquids. The secondary carpet backing usually is laminated to the stitched side, by bringing it and the stitched side of the tufted structure together with binder applied to the stitched side, or with binder applied both to the stitched side of the primary backing structure and to the secondary carpet backing, and curing the binder in contact with the stitched side and the secondary carpet backing. Curing, also commonly referred to as drying, typically involves heating the construction with hot air, as in a carpet finishing oven.
In carpet manufacture, the secondary carpet backing performs several functions. Typically, it imparts dimensional stability to the finished carpet and increases tuft bind. Dimensional stability is the ability of the carpet to resist buckling in use after stretch in. Generally dimensional stability increases when the force required to separate (or peel) the secondary carpet backing from the binder increases. Another carpet performance issue is ease of seaming. The secondary carpet backing must allow the carpet to be joined with seaming tape without distorting the face appearance while forming strong bonds that can withstand installation with a power stretcher. In addition to its contribution to carpet performance, the secondary carpet backing provides aesthetics and, in some cases, isolates the abrasive binder/filler mixture that is used to bind the stitches from the underside of the carpet. Isolating the abrasive binder/filler mixture is important for minimizing scratches to walls and woodwork during installation.
For commercially viable secondary carpet backings, the fabrics must also meet critical requirements in the carpet manufacturing process. For example, the backings must have sufficient integrity that they can be unrolled, coated and laminated to the back of the carpet without significant distortion of the pick line. They must also allow the carpet to be manufactured at line speeds that result in low laminating costs. One carpet backing property that correlates with faster line speeds is air permeability. Consequently, during curing (or drying) of the binder/filler mixture, the backing must not act as a barrier preventing escape of volatilized liquids. Carpets that have incompletely cured binders have less resistance to delamination, lower dimensional stability, and lower tuft binds, and lower resistance to moisture than equivalent carpets with fully cured binders.
These considerations dictate that the ideal secondary carpet backing must satisfy many different requirements. Among others, it must be capable of imparting dimensional stability, forming a strong bond to the carpet binder and to carpet seaming tape, allowing a fast carpet lamination process, and preventing the abrasive binder/filler mixture from causing damage.
Designing secondary carpet backings that meet all of these requirements is complicated, since the structural features that are conducive to good drying (i.e., an open structure) may be in conflict with the features for containing the filler and imparting increased dimensional stability. Studies performed by the inventors have shown that with the conventional leno weave secondary carpet backings that are in use today, it is not possible to predict with certainty the exact contribution of a secondary carpet backing to the dimensional stability of finished carpets because of the wide range of styles, weights and other characteristics of carpets in which secondary carpet backings are utilized. The inventors have also found the interactions of backings and binders within finished carpets when subjected to force are not well defined. Indeed, systematic studies of common secondary carpet backings have shown a lack of correlation between dimensional stability of finished carpets and the properties of the carpet backing fabrics therein, suggesting that the ability of such backings to impart dimensional stability is best gauged from carpet performance itself.
Leno weave is a weave in which the warp yarns are arranged in pairs which are twisted around the filling yarns. The most common secondary carpet backing in use today is an open weave fabric with a leno weave construction and a warp count of 16 ends per inch (wpi) and a fill count of 5 picks per inch (ppi). That product has polypropylene tape yarns in the warp direction and spun yarn in the filling direction. It combines low cost with the ability to provide adequate dimensional stability and delamination strengths in carpets that are manufactured at high line speeds. However, one drawback of using an open weave secondary carpet backing is that the binder/filler mixture is exposed on the underside of the finished carpet. This causes the back side of the carpet to have an unattractive appearance and abrasive texture due to the exposed filler/binder mixture. This abrasive, tough cured binder mixture often causes scratches on walls and woodwork when such carpets are installed.
FIG. 1 exemplifies a conventional leno weave construction. This conventional art does not constitute an admission of priority or that this figure is available as a reference against applicants' technology, or a waiver of any right the applicant may have under applicable statutes or Rules of Practice. As depicted, the construction 100 comprises warps 130 and wefts 120 disposed in a 16 warps per inch (wpi) by 5 picks per inch (ppi) leno weave construction. This provides a substantially open construction allowing for proper air permeability for drying of binding agents applied during carpet construction. For example, the open area of a 16×5 leno weave backing is about 34%. However, while allowing for proper air permeability, the open fabric design also allows the filler/binder mixture to exude from the back of the carpet. This creates an abrasive surface that is often unsatisfactory because of its appearance and tendency to scratch surfaces during installation.
Several approaches to overcoming the abrasiveness and poor aesthetics of the conventional leno 16×5 secondary carpet backing/filled binder system have been proposed. One approach is to increase the warp count and pick count of the 16 wpi by 5 ppi leno weave backing to about 18 wpi by 13 ppi. Although that approach reduces the size of the open areas in the fabric, the open areas are still sufficiently large that the filled binder mixture exudes through the secondary carpet backing during carpet manufacture.
Another approach is to make a secondary carpet backing that has a closed weave so that the filler/binder mixture is encapsulated between the stitched side of the primary backing and the side of the secondary carpet backing that is opposite the side that contacts the floor. One such product of that type is a 24 wpi×15 ppi plain weave fabric woven from 50 mil wide tapes in the warp direction and 40 mil diameter spun yarns in the filling direction. That fabric weighs about 4 ounces per sq yd (osy) and has essentially no open area, that is, no space in the plane of the fabric that is not occupied by a warp or fill yarn. Although that fabric prevents the filler/binder mixture from exuding to the underside of the carpet when it is cured, it does not allow for sufficiently high curing rates for widespread use. Consequently, it has been withdrawn from commercial use.
One key measure related to curing rates is the air permeability of fabric. For the above described 24×15 plain weave fabric, the air flow rate is 80 cfm/sq ft when measured according to ASTM D 737 with a 0.5 inch water head. That figure is about one tenth the air permeability of the conventional 16×5 leno weave secondary carpet backing described above.
Other approaches to improved secondary carpet backings have been proposed. One class of alternative secondary carpet backing constructions involves the preparation of composites of two or more layers. One such attempt is disclosed in U.S. Pat. No. 3,817,817 Pickens, Jr. et al. (“the '817 Pickens patent”) wherein a needlebonded secondary carpet backing for carpet comprises a closed weave fabric needlepunched with staple fiber. This composite backing affords a textile-like back surface that contains the filler/binder mixture, providing improved aesthetics and preventing the carpet from scratching walls and woodwork during installation. However, such a construction has low air permeability, and is not suitable for making carpets at current high line speeds because the water in the latex layer cannot escape at a sufficiently high rate while the carpet is in the oven.
FIG. 2 exemplifies the '817 Pickens patent. As is illustrated, the composite secondary carpet backing comprises warps 230 and wefts 220, wherein warps 230 and wefts 220 comprise thin, wide tapes or tape yarns. The fuzzy side of the fabric is disposed against the floor side of the backing/carpet construct, and the fibers that are punched through the woven fabric become bonded to the latex binder layer. However, the closeness and dimensions of these tape yarns in the '817 Pickens patent does not allow for sufficient water removal during the curing process. Additionally, the '817 Pickens patent does not teach a relationship between the structure of the woven fabric layer and subsequent performance in carpets.
Another composite secondary carpet backing is described in U.S. Pat. Nos. 6,060,145 and 6,344,254 Smith et al. These patents disclose a modified secondary carpet backing having an open weave scrim in a leno weave construction with a 16 wpi by 5 ppi count (16×5), wherein the woven scrim is bonded to a needled batt of fiber. The 16×5 open weave leno scrim described above is essentially the same as the 16 wpi by 5 ppi commonly used in secondary carpet backings as described above.
To create a less abrasive and more aesthetically pleasing carpet, the composite secondary carpet backings of the '145 and '254 patents combine the 16×5 leno construction with a needled batt of fiber. Like the Pickens, Jr. et al. '817 construction, this composite backing also affords a textile-like back surface providing improved aesthetics and preventing the carpet from scratching walls and woodwork during installation.
Thus, the '145 and '254 patents differ from the Pickens, Jr. et al. '817 construction because of the nature of the woven scrims. More specifically, the large open area in the woven scrim of the '145 and '254 patents leads to higher air flow through the composite backing. For example, with a 2.2 osy needled fiber layer, the air flow is about half that of the 16 wpi by 5 ppi fabric alone. This increased air permeability is cited as a key reason for a faster binder curing rate.
However, while the Smith et al. '145 and '254 patents provide increased air permeability over the Pickens, Jr. et al '817 construction, it is common industry knowledge that carpet laminations constructed with the '145 and '254 composite secondary carpet backings proceed at reduced line speeds compared to laminations where the 16×5 open weave fabric is alone the secondary carpet backing. Further, efforts to increase line speed by raising oven temperatures have been largely unsuccessful because the fibers in the nonwoven batt melt at the higher temperatures. These melted sections of the batt lead to poor aesthetics and a non-uniform carpet appearance and can also lead to decreased carpet properties.
Therefore, it is readily apparent that there currently exists a need in the art for a secondary carpet backings that allows for high carpet manufacturing speeds, provides a smooth textile-like surface that prevents the filler/binder mixture from exuding from the underside of the carpet, and imparts high levels of dimensional stability, delamination resistance, air permeability, and tuft binds to carpets.